We present near-infrared spectra for a sample of T dwarfs, including eleven
new discoveries made using the Two Micron All Sky Survey. These objects are
distinguished from warmer (L-type) brown dwarfs by the presence of methane
absorption bands in the 1--2.5 um spectral region. A first attempt at a
near-infrared classification scheme for T dwarfs is made, based on the
strengths of CH4 and H2O bands and the
shapes of the 1.25, 1.6, and 2.1
um flux peaks. Subtypes T1 V through T8 V are defined, and spectral
indices useful for classification are presented. The subclasses appear to
follow a decreasing Teff scale, based on the evolution of
CH4 and
H2O bands and the properties of L and T dwarfs with known distances.
However, we speculate that this scale is not linear with spectral type for
cool
dwarfs, due to the settling of dust layers below the photosphere and
subsequent
rapid evolution of spectral morphology around Teff ~ 1300--1500 K.
Similarities in near-infrared colors and continuity of spectral features
suggest that the gap between the latest L dwarfs and earliest T dwarfs has
been
nearly bridged. This argument is strengthened by the possible role of CH4
as
a minor absorber shaping the K-band spectra of the latest L dwarfs. Finally,
we
discuss one peculiar T dwarf, 2MASS 0937+2931, which has very blue
near-infrared colors (J-Ks = -0.89+/-0.24) due to suppression of the
2.1
um peak. The feature is likely caused by enhanced collision-induced
H2 absorption in a high pressure or low metallicity photosphere.